Within a printing unit, the print substrate is moved along a conveyor operation sequence by various transports in order to be supplied to individual stations within the printing unit. Here the substrate stock can then be printed, or other desired processes can be carried out on it. Thus, transport of the substrate stock can take place with or without grippers. Where transport takes place without grippers, e.g. in digital printing units, rollers or belts can be used to hold the substrate stock down and transport it by electrostatic force or through vacuum fixtures.
If a substrate abandons the planned conveyor operation sequence, then it can do damage in other areas of the machine. The danger of such false runs exists particularly in the case of transport without grippers. Therefore, various processes are used to prevent or at least to recognize such false runs promptly so that the machine, or at least the area of the machine where the substrate stock deviates from the planned conveyor sequence, can be stopped before damage occurs. This is especially true for S-shaped substrate stock.
In EP 0 916 602 A 1, comparing the number of substrates fed into the machine with the number recognized by sensors inside the machine, is proposed for recognizing false substrate runs. In that case, sensors are in the environment of the planned conveyor operation sequence. Where the number of substrates sensed inside the machine is too small, it is concluded that there is a false substrate, an alarm is triggered, and appropriate countermeasures are taken. The number of substrates supplied is transmitted by a control signal to a monitoring device planned for that purpose.
If light-sensitive sensors are used to recognize a substrate stock, then substrate stocks can be erroneously identified because of impurities of the transport. It is possible that the absence of substrate stock and therefore a possible false run are not recognized, and damage can occur. Because of impurities, a zone with appropriate change with reference to the reflective action can exist on the surface itself, whereupon a substrate stock can be erroneously identified. In order at least to limit this source of error, the transport must at least be regularly cleaned in order to limit impurities.
With the device above, substrate stocks can be recognized only to the extent that their capacity for reflection, measured by the range of the light wavelength collected by the sensor, differs very distinctly from that of the transport or generally from that of the surface on which the substrate stock is to be recognized. With this device, if the control signal fails, for example, when the machine restarts, then the recognition of false substrates does not operate and it has to be switched off for at least a short period during the re-start. During this time period when the danger exists that unrecognized false substrates could cause damage.
In the German utility model G 82 15 605, a locking device for rotary printing units is presented that can recognize substrate stocks by a light-intensive sensor in the environment of a blanket cylinder. In this connection it is contemplated that a substrate stock, which abandons the planned conveyor operation sequence can be recognized at sites outside the planned conveyor sequence by a light sensitive sensor.
By means of the sensor recommended in G 82 15 605, a false run is recognized in that the reflective capacities of substrate stock and surface of the blanket cylinder can be differentiated within the range of the light wavelength where the sensor is sensitive. The reflected light is measured by the sensor and the presence of a substrate stock can be concluded where there is a sufficient change of reflected luminosity, and appropriate protective measures can be initiated. At this point the total luminosity is measured across the range of the light wavelength measurable by the sensor. This range of light wavelength can typically encompass the total visible spectrum.
If the substrate stock and the blanket cylinder show a similar mean reflective capacity in the range of light wavelength measurable by the sensor, then it is not possible to recognize a difference between the substrate stock and the blanket cylinder. A false run of a substrate stock can reach the area behind the sensor without being detected and cause damage there.
Contamination of the blanket cylinder also represents a source of error. If too much contamination is present on the surface of the cylinder, then it is possible that the difference in the total luminosity measured is sufficient to identify a substrate stock erroneously, and a false alarm will be triggered. Frequent cleaning of the blanket cylinder is therefore also necessary here.